Cyclonic separating apparatus is well known and has uses in a wide variety of applications. Over the last decade or so, the use of cyclonic separating apparatus to separate particles from an airflow in a vacuum cleaner has been developed and introduced to the market. Detailed descriptions of cyclonic separating apparatus for use in vacuum cleaners are given in, inter alia, U.S. Pat. No. 3,425,192, U.S. Pat. No. 4,373,228 and EP 0 042 723. From these and other prior art documents, it can be seen that it is known to provide two cyclone units in series so that the airflow passes sequentially through at least two cyclones. This allows the larger dirt and debris to be extracted from the airflow in the first cyclone, leaving the second cyclone to operate under optimum conditions and so effectively to remove very fine particles in an efficient manner. This type of arrangement has been found to be effective when dealing with airflows in which is entrained a variety of matter having a wide particle size distribution. Such is the case in vacuum cleaners.
It is also known to provide cyclonic separating apparatus in which a plurality of cyclones are arranged in parallel with one another, as in, for example, U.S. Pat. No. 2,874,801. Furthermore, it is known to provide such a plurality of parallel cyclones downstream of a single cyclone, as in, for example, U.S. Pat. No. 3,425,192. However, the entries to these parallel cyclones are commonly via a plenum chamber with which the inlets to the parallel cyclones communicate in a direct manner. Other arrangements of parallel cyclones include uniform ducts leading from a plenum chamber to the inlet of each cyclone: see, for example, U.S. Pat. No. 3,682,302.
The passage of the air through a plenum chamber often causes unnecessary pressure losses because the relatively small inlets to the parallel cyclones bring about sudden and quite dramatic changes in the cross-section of the airflow path along which the air is flowing. The overall efficiency of the cyclonic separating apparatus is therefore lower than necessary.